In many electro-optical or photoelectric devices, it is necessary to transmit a charge through a coating or to dissipate an electric current. In these applications, a transparent conductive coating is used. For example, transparent conductive coatings are an essential element in liquid crystal displays, panel displays, photovoltaic cells, electrochromic devices, electroluminescent lamps and flat panel televisions or spectroelectrochemical analytical devices.
According to Porter, “Measurements: Optically Transparent Carbon Electrodes”, Analytical Chemistry, January, issue 1, pp. 14-22 (2008), the most common transparent conductive electrode material for electro-optical devices and photovoltaics is indium tin oxide (ITO) and zinc Oxide ZnO. ITO is an opaque semiconductor that must be applied in thin layers (e.g. layers having a thickness of about 0.1 microns or less) to remain transparent. Indium is a very expensive metal and there is a potential shortage in the world's supply. Broader usage of optical flat panels and screens requires new and more reliable conductive coating. In spectro-electrochemistry applications, a flat electrode can serve for reductive or oxidative reactions in a liquid media or as an optically transparent window for Ultraviolet visible (UV-vis) or Infrared (IR) light spectroscopy. Few carbon materials have been used as optically transparent conductors (OTEs) in spectroelectrochemistry
According to DeAngelis in Analytical Chemistry, Vol. 49, No. 9, pp. 1395-1398, August (1977), carbon materials combine four attractive features useful in these types of application, for example:
(1) A wide electrical potential window which extends into both the positive and negative regions of the potential versus a saturated calomel electrode. The carbon electrode exhibits low residual current throughout a variation of the potential between −1V and 1V in aqueous media (pH 10 carbonate-borate buffer, 0.5 M KNO) at a scan rate of 20 mV/s. The carbon film appears to be very stable under conditions of repeated potential cycling in aqueous media of varying pH.
(2) A good electrochemical activity measured as the electron transfer rate constant ks or k0 being higher than 5.0×10−5 cm/s for a range of redox systems. Of particular relevance to the current review are oxidations and reductions of organic and biological molecules in both aqueous and non-aqueous media, for which the electrochemical properties of carbon electrodes are often superior to those of noble metals (McCreery, Chemical Reviews, 2008, Vol. 108, No. 7, p. 2646).
(3) A chemical stability (relatively inert carbon surfaces do not react with other chemicals when placed in contact with each other) under strongly acidic (e.g., acidity is measured by the pH value, pH from 0 to 7 is acidic and a pH of from 7 to 14 is basic, a pH of 5 or below is strongly acidic) and alkaline conditions.
(4) A wide array of well-known strategies for surface modification for instance Chen et al., Analytical Chemistry, Vol. 68, No. 22, pp. 3958-3965 (1996), reports that “various well-established and novel surface modification procedures were used on glassy carbon (GC) electrodes to yield surfaces with low oxide content or which lack specific oxide functional groups.” Furthermore, McCreery, Chemical Reviews, 2008, Vol. 108, No. 7, pp. 2646-2687, reports that new approaches to surface modification based on radical and photochemical reaction such as electrochemical reduction of phenyl diazonium reagents, oxidation of primary amines to a radical, or cycloaddition chemistry have significantly broadened the utility of carbon electrodes by providing very stable surface modifications.
However the design of a carbon film-based OTE involves a trade-off between optical transparency and electric resistivity, both of which are functions of the film thickness. According to US 20100187482 A1: low electrical resistance and high optical transparency are oppositely influenced by the film thickness.
A number of chemistries and processes have been tested for the past decades. For example, Kummer et al., Analytical Chemistry, Vol. 65, No. 24, pp. 3720-3725 (1993), investigated the preparation of a carbon optically transparent electrode by the deposition of thin films of carbon onto metal mesh substrates. Carbon coatings were produced by either spray coating Acheson DAG® 40 colloidal graphite suspended in butyl acetate onto a metal mesh at room temperature or by pyrolysis of acetone on a resistively heated metal mesh. The mesh provided the transparency of the electrode.
In the case of Sorrels et al., Sorrels, Analytical Chemistry, Vol. 62, No 15, pp. 1640-1643 (1993), a thick (0.5 to 3.5 mm) a reticulated vitreous carbon electrode (fabricated by Electroanalysis, Inc.) is sliced from a porous, vitreous carbon foam material. Further, Mattson et al., J. Mattson et al., Analytical Chemistry Vol. 47 No. 7, pp. 1122-1125 (1975), uses ultra high purity glassy carbon that is evaporated using an electron beam technique and deposited on glass and quartz substrates to make carbon optically transparent electrode.
According to Anjo, Anjo et al., Analytical Chemistry, Vol. 65, No. 3, pp. 317-319 (1993), carbon films are attractive because the film is deposited directly on the optical components; this places the electrode directly on the optical window and does not take up volume in the electrochemical cell. Anjo prepares a carbon film on a quartz substrate by a vacuum pyrolysis of 3, 4, 9, 10-perylenetetracarboxylic dianhydride. The carbon source 3, 4, 9, 10-perylenetetracarboxylic dianhydride is sublimed and then vapor-pyrolized at 800° C. on the surface of a quartz substrate producing a mirror-like conductive coating with a transmittance varying between 1% and 32% depending on the duration of the deposition.
EP1063196 (2008) discloses a carbonaceous complex structure comprising a layered set of a substrate, a carbonaceous thin film and a fullerene thin film. The film is obtained by thermally decomposing a carbon compound such as fullerene molecules or organic solvents, such as ethanol or toluene. The conductivity of the carbonaceous film is in the order of 0.01 S/cm. Such a low conductivity is not suitable for a transparent electrode in optoelectronic devices, such as solar cells.
Donner et al., Analytical Chemistry, Vol. 78, No. 8, 2816-2822 (2006) describe the preparation of carbon-based optically transparent electrodes by pyrolysis in a reducing atmosphere of thin films of photoresist AZ@ 4330-RS after it is spin coated onto quartz substrates. The photoresist AZ@ 4330-RS from Clariant is a cresol-novolak resin with highly branched structures and the reaction of this polymer with diazonaphthoquinono-sulfonic esters results in a hard amorphous carbon structure. The films obtained by this course of action show low transparency of only 47% for a 13 nm thick carbon film which might not meet requirements in applications such as optoelectronic devices.
Lee et al., Appl. Mater. Interfaces, 2009, 1 (4), pp 927-933, discloses spin coating and pyrolyzing thin layers of Durez Furfuryl alcohol resin no. 16470 (Occidental Chemical Corp.) Koppers Inc. coal tar pitch and Shipley S1805 photoresist (PR) into thin 3 to 6 nm carbon layers on glass substrates. Schreiber, Applied Surface Science, Volume 256, Issue 21, 15 Aug. 2010, Pages 6186-6190, discloses spin coating and pyrolyzing various photoresists, e.g. PMMA, SU8, AZ nLOF 2070, AZ nLOF 4533, ma-N 20401 from MicroChemicals GmbH, Ulm, Germany.
US 20100187482 A1 describes the production of a thin highly transparent and conducting carbon film by spin coating a solution of a discotic precursor and heating the substrate from 400° C. to 2,000° C. The carbon film obtained, according to the process of US 20100187482 A1, has a higher thermal and chemical stability than traditionally used ITO. Further, carbon film has an extremely smooth surface, which cannot be obtained for example with carbon nanotube films. US 20100187482 A1 describes films having both a high transparency and at the same time a low electrical resistance. Depending on the thickness of the carbon film of US 20100187482 A1, the resistivity of the carbon film of US 20100187482 A1 is between 10−5 Ω·m and 10−8 Ω·m.
According to US 20100187482 A1:
“ . . . a compromise between electrical resistance and optical transparency had to be accepted with all known methods due to their dependence on the carbon film thickness. Generally, resistance of carbon films undergoes a dramatic increase as thickness decreases below around 30 nm. Therefore, hitherto reported carbon films even in the thickness of .about. 13 nm, with sheet resistance in the range of 1000-2000 ohm/sq, have transmittance lower than 55%. Since these reported carbon film electrodes were only used in spectroelectrochemical studies, such transparency was enough. However, such low transparency cannot meet demand of modern devices such as optoelectronic devices. Besides high transparency, modern devices require transparent electrodes with low resistance, smooth surface as well as suitable work function which depends strongly on the structure of carbon film. Obviously, the type of precursor and preparing methods are important for fabrication of structure-controllable carbon films. Furthermore, most of the reported methods for preparing transparent carbon films are complicated.”
A problem remains in the industry to develop a low viscosity formulation that can be cured and carbonized in a layered structure for example in a thin layer (e.g. 1 micron in thickness or less) with high light transmittance and high conductivity. Therefore, it would be an advance in the art to prepare a suitable precursor and a simple procedure for making a highly transparent, conductive and structure-controllable carbon film with a smooth surface and an appropriate work function for modern device applications, particularly for use in optoelectronic devices.